Generator fault protection



GENERATOR FAULT PROTECTION I Filed Aug. 19, 1945' ATTOR N EY Patented Aug. 19, 1947 GENERATOR FAULT PROTECTION Donald w. Exner, Lima, om, assignor in westinghouae Electric Corporation, East Pittsburgh, Pa., a corporation of Penmylvania Application August 19, 1943, Serial No. 499,189

The present invention relates to a protective relay system for protection against internal faults in alternating-current, or constant-speed directcurrent, generators and, more particularly, to such a system which is especially adapted for use with aircraft generators.

The protective relay system of the present invention is especiallydntended for-use with generating systems for use on aircraft, although its usefulness is not necessarily restricted to this particular application. Alternating-current aircraft generating systems consist of an alternating-current generator, an exciter, or other means for supplying direct-current excitation to the generator field, and a voltage regulator controlling either the exciter field current or the generator field current to maintain the generator voltage substantially constant. The invention is also applicable to some types of direct-current aircraft generating systems, consisting of a direct-current generator driven at constant speed, a separate exciter. and a voltage regulator. The occurrence of an internal fault in the generator of such systems may be very dangerous, since the generator terminal voltage drops upon the occurrence of a fault and the voltage regulator increases the field current, thus tending to increase the current flowing into the fault. In this way,

a bad fire may be started unless the generator excitation is promptly removed.

The usual methods for protection against internal generator faults, used for other applications, involve the use of some type of differential relaying system. Such systems, however, require the use of relatively heavy and bulky relays which are unsuitable for use on an airplane, where the Claims. (01. 171-312) space is limited and the weight must be kept to a minimum. Differential relaying systems are also relatively expensive, especially as compared to the cost of generators of relatively small physical size, such as are used on aircraft. For this reason, the conventional types of protective systerns, which are usually used for protection against internal generator faults, are not suitable for use on aircraft, and some lighter and simpler system must be used, even though its response may be somewhat slower.

The principal object of the present invention is to'provide a protective relaying system, for protection against internal generator faults, which is simple and light in weight so as to be especially well suited for use with aircraft generators.

A further object of the invention is to provide a protective relay system for protection against internal faults in alternating-current or con- 2 stant-speed direct-current generators provided with a voltage regulator, the relay system operating in response to a condition of maximum or excessive current in the excitation system of the generator.

Another object of the invention is to provide a protective relay system for protection against internal generator faults, using a, simple lightweight time-delay current or voltage responsive relay which operates to remove the excitation from the generator in response to a condition of maximum or excessive current in the excitation system.

A more specific object of the invention is to provide a protective relay system for protection against internal generator faults, using a simple thermal relay having a heater element which is energized by, or in response to, the current in the excitation system of the generator, and which has contacts connected to effect dee'nergization oi the excitation system in response to heating caused by maximum or excessive current in the excitation system.

The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawing, in

which:

Figure 1 is a schematic wiring diagram showing one embodiment of the invention; and

Fig. 2 is a similar diagram showing another embodiment of the invention.

One embodiment of the invention is shown in Fig. 1 in connection with an alternating-current generator 1, which may be of any type. The generator I is driven by any suitable prime mover which, in an aircraft application, is usually the aircraft engine, the generator being preferably driven at a constant speed -from the engine in any suitable manner. The generator I has a threephase armature winding 2, which is connected-to the load circuit by means of a circuit breaker 3. The circuit breaker 3 is shown as being of the latching type, having a closing coil 4 which moves the breaker to closed position when energized, and a latch '5 which latches the breaker in its closed position. A trip coil 6 is provided to release the latch and permit the breaker to open.

The generator I has a field winding 1 which is supplied with direct current by an exciter having an armature 8 and a self-excited field winding 9. The excitation system of the generator also includes a voltage regulator l0, which has been shown diagrammatically as including a variable resistance l I, connected in series with the exciter field 9, and an operating coil l2; connected to a direct-current bus 23.

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three-phase rectifier II. The rectifier I3 is connected across the terminal voltage of the generator I, so that any change in the generator voltage affects the energization of the coil I2, which effects a corresponding adjustment in the value of the resistance II to change the excitation and restore the voltage to its correct value. The voltage regulator Ill preferably also include a damping transformer I4, having its primary winding connected across the generator field I and its secondary winding connected in series with the coil I2. The voltage regulator, as such,

forms no part the present invention, and any suitable type of regulator may be used.

The circuit of the exciter field winding 3 is controlled by an exciter field relay II, which has an operating coil I3, and a contact I1 connected in the exciter field circuit, so that when the coil I3 is energized, the contact II closes and completes the circuit of the exciter field winding 3. The exciter field relay l 5 also has a holding contact I3 which is connected to complete a holding circuit when it closes, to maintain the energization of the coil I3. The coil I3 of the relay I3 is initially energized by means of a manually operated switch-arm I3, which is preferably of a type that is returned to its open position as soon as it is released, by a spring or other means. The switch-arm I9 is connected to a direct-current bus 23, which is supplied from any suitable source of direct current, indicated diagrammatically as a battery 2I, one terminal of which is connected to the bus 23 and the other terminal grounded at 22.

The switch-arm I 9 is adapted to be placed alternatively on either 01 two fixed contacts 23 and 24. The contact 23 is connected to the coil I6 of the relay I5, so that when the switch-arm I9 is placed on the contact 23, a circuit is completed from the direct-current bus 23 through the coil I3 to a ground connection 23, energizing the coil It. When the relay I 3 closes its contacts I1 and I3 in response to energization of the coil-i6, the exciter field 9 is connected to the exciter armature 8 by the contact I1, and the contact I8 completes a holding circuit from the direct-current bus 20 through the conductors 23 and 21 to the coil I6, so that the coil remains energized after the switch-arm I9 returns to its open position.

The contact 24 is connected to the operating coil 28 of a tripping relay 23, which has a normally-closed contact 33 connected in the holding circuit for the coil 16. Thus, when the switcharm I 9 is placed on the contact 24, the coil 28 is energized and the contact 33 is opened, interrupting the holdin circuit for the coil I3, and permitting the relay I5 to open its contacts.

The exciter field relay I5 preferably also has an additional normally-open contact 3i and a normally-closed contact 32 for the control of the circuit breaker 3. The circuit breaker 3 is actuated by a manually operated switch-arm 33, which is preferably of the same type as the switch-arm I9, having a, spring or other means for returning it to the open position as soon as it is released. The switch-arm 33 is adapted to be placed alternatively on either of two fixed contacts 34 and 35, and it is connected to the The contact 34 is connected in series with the normally-open contact 3I of the relay I3 and the closing coil 4 of the circuit breaker 3, the other end or the coil 4 being grounded, as indicated at 36. The normallyclosed contact 32 of the relay I5 is connected in series between the direct-current bu 23 and the trip coil 3 of the circuit breaker 3, and the contact 33 is also connected to the trip coil 3. The other end 01' the trip coil 3 is connected to ground at 33, the connection being made through an auxiliary contact 31 on the circuit breaker 3, which is open when the breaker is open.

Protection against internal faults in the generator I is provided bymeans of a thermal relay 33. Any type of time-delay current-responsive relay might be used for this purpose. but a simple thermal relay is preferred, sinceit is the lightest in weight of any available type oi relay which is suitable for the purpose. The relay 33 is shown as having a heating coil 39, which is connected in series in the circuit of the exciter field winding 3, and a. thermally-responsive element 43, which may be a bimetallic disc, adapted in its closed position to bridge a pair of fixed contacts 4I. The contacts H are connected, as shown, in the holding circuit of the relay II. The thermallyresponslve relay 38 is calibrated so that it does not respond to the normal exciter field current, or to the field currents corresponding to normal or expected generator overloads, but it is responsive to a sustained condition 01' maximum current in the exciter field circuit or, more generally, to an excessive exciter field current which is greater than the current corresponding to a permissible generator overload. The thermally-responsive element 43 is designed to fiex out of engagement with the contacts 4|, and thus interrupt the holding circuit of the relay I3, when it is heated above a predetermined temperature by excessive exciter field current flowing through the heater coil 39. The relay 33 is preferably mounted in a position where it is not exposed to direct heating from the generator or exciter, and if it is necessary, in a particular case, to locate the relay 33 in a position where it is subjected to such heating, a temperature-compensated type of relay should be used, so that it will not be afi'ected by heat received directly from the generator or exciter.

The operation or this system is as follows. Assume that the generator I is disconnected from the load circuit, with the circuit breaker 3 open, and the relays I3 and 29 in their deenergized positions, as shown on the drawing. If the generator I and exciter are being driven by the prime mover at the proper speed, the generator I can be connected to the line. In order to do this. the field 1 is first excited by placing the switcharm I3 on the contact 23. This energizes the coil I3, as previously explained, and-causes the exciter field relay II, to close its contacts I1, I3 and 3 I, and to open the contact 32. The switcharm I9 may be released as soon as the relay II has operated, since a holding circuit is completed by closure of the contact I! extending from the direct-current bus 20 through the conductor 23, the normally closed contact 33 of the relay 29, the contracts 4i and bimetallic disc 43 of the relay 33, the-contact I3, conductor 21 and coils I6 to ground connection 25. Thus, the relay I5 remains in its actuated position. Closure oi the contact I! of the relay I 5 completes the circuit for the exciter field winding 3, so that the field winding 1 of the generator I is energized from the exciter 3, to which it is preferably permanently connected. The circuit breaker 3 can now be closed by placing the switch-arm 33 on the contact 34, which completes a circuit from the direct-current bus 20 through the contact 3I of the relay 15, which is now closed, to the closing to the load and the system is in its normal running condition.

If an internal fault occurs in the armature winding 2 of the generator I, the generator terminal voltage will immediately drop to substantially zero, or at least to a relatively low value. The voltage regulator I3 will attempt to-restore the voltage to its normal value by increasing the excitation, and will adjust the variable resistance II to its lowest value, thus producing a condition of ceiling current, or maximum current, in the exciter field circuit and in the generator field circuit. This excessive current in the exciter field circuit increases the heating effect of the heater coil 33 so that the thermally-responsive element 40 is heated to the temperature at which it operates to interrupt the circuit through the contacts II. In this way, the holding circuit of the relay I5 is interrupted and the coil I3 is deenergized, permitting the relay I3 to drop out and open the contact II, interrupting the exciter field circuit and thus removing the excitation from the generator I, so that the dangerous condition of excessive current flowing into the internal fault is prevented.

When the relay I5 drops out, opening the contacts II, I3 and 3|, the contact 32 closes and completes acircuit from the direct-current bus 20 through the trip coil 6 of the circuit breaker 3 and the auxiliary contact 31, which is closed when the breaker is closed, to the ground connection 33. The trip coil 3 is, therefore, energized, and the breaker 3 is tripped to disconnect the generator from the load circuit, the circuit through the trip coil 3 .being interrupted by opening of the auxiliary contact 31 when the breaker opens. Thus, the protective system operates automatically in response to excessive current in the excited field circuit to remove excita-. tion from the generator and to trip the circuit breaker 3.

In normal operation, the generator I can be disconnected from the line by placing the switcharm I3 on the contact 24. This energizes the coil 23 of the relay 23 and causes it to open its contact 30, thus interrupting the holding circuit of the relay I5. The relay I3 then drops out, effecting deenergization of the exciter field winding 3 and tripping of the circuit breaker 3 in the manner described above, If it is desired to disconnect the generator I from the load circuit without removing the excitation, this can be done by placing the switch-arm 33 on the contact 33 which energizes the trip coil 8 directly from the direct-current bus 23, and trips the circuit breaker 3 without affecting the relay I5, so that the breaker can be tripped without removing excitation from the generator I.

Thus, it will be seen that by the use of the thermal relay 33 a simple but effective protective relay system is provided for removing excitation from the generator I in case of an internal fault, and preferably also for tripping the circuit breaker 3 to disconnect the generator from the load circuit. Various modifications of this system are, of course, possible within the scope of the invention. Thus, the control of the circuit breaker 3 by the relay I! can be eliminated in some cases, although it is usually desirab e, and is necessary if the generator I is operat ng in parallel with oneor more other generators.

The heater element 33 of the thermal relay 33 has been shown cdiinected in the exciter field circuit, but it willilbe apparent that it could equally well be connected in the circuit of the generator field winding I so as to be responsive to the generator fieldcurrent. It is usually pref erable,however, to connect it in the exciter field circuit because a higher ratio is obtained between the maximum or ceiling exciter field current, when the relay must operate, and normal overload field current when the relay should not operate. This higher ratio in theexciter field circuit is the result of magnetic saturation in the generator and exciter. The generator field current increases at a faster rate than the load current because of the shape of the saturation curve, but the exciter field current rises still more rapidly with respect to the exciter armature current, which is substantially equal to the generator field current, because the excitervoitage rises as its armature current increases. For this reason, a higher ratio of maximum current tooverload current is,obtained in the exciter field circuit, and it is therefore preferred to connect the relay heater 33 in this circuit, since it simplifies the design of the relay and enables a faster tripping time to be obtained, as well as enabling the relay to distinguish morereadily between overload conditions and fault conditions. Another advantage of placing the relay in the exciter field circuit is that the current is smaller than the current in the generator field circuit, so that the relay can be made smaller and lig ter. The system will operate in the same ma, ner, however, if the heater coil 33 is designed to be connected in the generator field circuit, and such an arrangement is 'within the scope of the invention.

The thermal relay 38 has been shown and described as having its contacts connected in the I holding circuit for the exciter field relay I5. If a type of thermal relay is used in which the contacts remain open after operation until they are manually reset, the contacts might be connected directly in the exciter field circuit, or in the generator field circuit, and in some cases this might result in some simplification in the control system. Such a relay, however, is somewhat more expensive than the type shown in the drawing,

which recloses its. contacts as soon as it has shown in Fig. 2, which shows the use of a voltageresponsive relay instead of a current-responsive relay. In this figure, the generator I and exciter 3 may be the same as in Fig. l, with a voltage regulator I3 connected in the same manner as in Fig. 1. The generator I is connected to a load circuit by means of a circuit breake 43', which may be operated manually, or in any suitable manner. The circuit of the exciter field winding 3 is controlled by an exciter field relay 46, having an operating coil 41, and a contact 43 connected to complete the circuit or the exciter field winding 3. The coil 41 is'energized by means of a switch-arm 43 which maybe similar to the switcharm I3 of Fig. 1, and which'is adapted to be The switch-arm 48 is connected to the direct-current bus 20, and when placed on the contact 50 completes an obvious circuit through the coil 41 to the ground connection '62. The relay 4. also has a holding contact 53 which completes a holding circuit when closed to maintain the energization of the coil 41. The contact 5| is connected to the operating coil 54 a tripping relay II which has a normally-closed contact 58 connected in the holding circuit for the relay 46.

Protection against internal generator faults is provided in this embodiment of the invention by a thermal relay which has a heater coil 58 and a thermally-responsive element 59 arranged to normally bridge a pair of contacts 80. The contacts 60 are connected in series with the contact N in the holding circuit for the relay 48. The heater coil 58 is'a voltage-responsive coil, and is connected across the exciter field winding 9 so as to be responsive to the voltage across it, which is, of course, proportional to the exciter field current. Since the coil 58 is connected across the exciter field winding 9, it carries a current which is proportional to the current in the field winding 9, and the relay I1 is calibrated to operate in response to heating of the coil 58 corresponding to maximum current in the exciter field winding 9, or to an excessive current which is greater than the exciter field current corresponding to any permissible normal overload of the generator I.

It will be apparent that the operation of this embodiment of the invention is similar to that described above in connection with Fig. 1. Thus, the generator field 1 is energized by placing the switch-arm'49 on the contact 50, which completes a circuit through the coil 41 to the ground connection 52, thus energizing the coil 41 and causing the relay 48 to close its contacts 48 and 53. Closing oi the contact 48 completes the exciter field circuit and effects energization oi' the generator field 1, since the generator field is connected to the exciter, while closing of the contact '53 completes an obvious holding circuit for the relay 46 to maintain the coil hold the contact 48 closed.

In case of an internal fault in the generator, the voltage regulator i0 operates in the manner described above to increase the exciter field current to its maximum value, and this causes the thermally-responsive element 59 of the relay 51 to operate to interrupt the holding circuit for the relay 48, permitting the relay to drop out and deenergize the exciter field winding.

In normal operation, the excited field winding can be deenergized by moving the switch-arm 48 into engagement with the contact 5|, which energizes the coil 54 of the tripping relay 55 and 41 energized and causes it to open its contact 56, interrupting the holding circuit for the relay 46. The circuit breaker 45 may be operated in any desired manner, either manually or by any suitable control system, such as that shown in Fig. l.

The heater coil 58 of the thermal relay 5?, in the embodiment of Fig. 2, may be connected across the generator field winding I rather than the exciter field winding 9 if desired, but it is preferred to connect it across the exciter field winding because of the higher ratio of maximum field current to overload field current obtainable in the exciter field circuit, as compared to the corresponding ratio in the generator field circuit, as explained above in connection with Fig. 1. The heater coil 58 may, however, be connected in either position within the scope of the invenauxiliary control circuit, as shown, when a thermal relay of the automaticreclosing type is used in order to avoid cycling or the system.

It should now be apparent that a simple and reliable protective relay system has been provided for the protection of electric generators against internal faults, and that this system is particularly adapted for use in connection with aircraft generators because oi its use of a simple light-weight relay. The generator I has been shown and described as a three-phase alternating-current generator, but the invention is also applicable to other types of generators, such as single-phase generators, and direct-current generators driven at constant speed and provided with separate exciters.

It will be understood, 01' course, that the invention is capable of various modifications, some of which have been indicated above, and that it is not limited to the particular details shown and described for the purpose of illustration. Thus, any type of time-delay currentor voltageresponsive relay might be used, although a thermal relay is preferred because of its light weight and simplicity. Similarly, various other changes and modifications might be made within the scope of the invention, and it is to be understood, therefore, that the invention is not limited to the particular arrangement shown, but in its broadest aspects it includes all equivalent modifications and embodiments which come within the scope of the appended claims.

I claim as my invention:

1. In combination, a generator, a field winding for said generator, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for effecting an increase in current in said exciter field winding in response to a drop in the generator voltage, and means responsive only to continued excessive total current in the exciter field winding for effecting deenergization of the exciter field winding.

2. In combination, a generator, a field winding for said generator, an exciter connected to supply direct current to said generator field winding, 1!. field winding for said exciter, means for eflectlng an increase in current in said exciter field winding in response to a drop in the generator voltage, and time-delay relay means responsive only to excessive total current in the exciter field winding for effecting deenergization of the exciter field winding.

3. In combination, a generator, 9. field winding for said generator, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for efiecting an increase in current in said exciter field winding in response to a drop in the generator voltage, and thermally-responsive relay means having a heating element connected in series with the exciter field winding and having contact means adapted to be actuated in response to heating caused by excessive current in the exciter field winding, said contact means being operative when actuated to effect deenergization of the exciter field winding.

4. In combination, a generator, 9. field winding for said generator, a circuit breaker for connecting said generator to a load circuit, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for rent in said exciter field winding in response to a drop in the generator voltage, and thermallyresponsive relay means having a heating element connected in series with the exciter field winding and having contact means adapted to be actuated in response to heating caused by excessive current in the exciter field winding, said contact means being operative when actuated to effect deenergization of the exciter field winding, and trippingof said circuit breaker.

5. In combination, a generator, a field winding for said generator, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for effecting an increase in current in said exciter field winding in response to a drop in the generator voltage, and thermally-responsive relay means having a heating element connected across the exciter field winding, and having contact means adapted to be actuated vin response to heating caused by excessive current in the exciter field winding, said contact means being operative when actuated to effect deenergization of the exciter field winding.

6. In combination, a generator, a field winding for said generator, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for effectefiecting an increase in curing an increase in current in said exciter field winding in response to a. drop in the generator voltage, thermally-responsive relay means having a heating element connected in series with the exciter field winding and having contact means adapted to be actuated in response to heat from said heating element, a relay having contacts connected in the circuit of the exciter field winding, and means for energizing said last-mentioned relay to cause it to close its contacts to effect energization of the exciter field winding, said relay also having contacts for completing a holding circuit for maintaining the relay energized, the contact means of said thermally-responsive relay being also connected in said holding circuit and being adapted to interrupt said circuit in response to heating caused by excessive exciter field winding current.-

7. In combination, a generator, a field winding for said. generator, a circuit breaker for connecting said generator to a load circuit, an exciter connected to supply direct current to said generator field winding, a. field winding for said exciter, means for effecting an increase in current in said exciter field winding in response to a drop in the generator voltage, thermally-responsive relay means having a. heating element connected in series with the exciter field winding and having contact means adapted tobe actuated in response to heat from said heating element, a relay having contacts connected in the circuit of the exciter field winding, and means for energizing said last-mentioned relay to cause it to close its contacts to exciter field winding, said relay also having 'contacts for completing a, holding circuit for maintaining the relay energized and for-effecting control of said circuit breaker, the contact means of said' thermally-responsive relay being also connected in said holding circuit and being adapted to interrupt said circuit in response to heating caused by excessive exciter field winding current.

effect energization of the 8. In combination, a generator, 9. field winding for said generator, an exciter connected to supply direct current to said generator field winding, a field winding for said exciter, means for effecting an increase in current in said exciter field winding in response to a drop in the generator voltage, thermally-responsive relay means having a heating element connected across the exciter field winding and having contact means adapted to be actuated in response to heat from said heating element, a relay having contacts connected in the circuit of the exciter field winding, and means for energizing said last-mentioned relay to cause it to close its contacts to eiiect energization of the exciter field winding, said relay also having contacts for completing a. holding circuit for maintaining the relay energized, the contact means of said thermally-responsive relay being also connected in said holding circuit and being adapted to interrupt said circuit in response to heating caused by excessive exciter field winding current.

9. In combination, a generator, an excitation system for supplying field excitation to said generator, said excitation system including a. generator field winding and means for supplying direct-current excitation to said field winding, means responsive to the generator voltage for effecting an increase in the current in said excitation system in response to a decrease in generator voltage, and thermally-responsive means having a heating element energized in response to the total current in a part of the excitation system, said thermally-responsive means being adapted to effect deenergization of the excitation system when heatedin response to excessive current in said part of the excitation system.

10. In combination, a generator, an excitation system for supplying field excitation to said generator, said excitation system including a generator field winding and means for-supplying direct-current excitation to said field winding, means for connecting said generator to a load circuit, means responsive to the generator voltage for effecting an increase in the current in said excitation system in response to a' decrease in generator voltage, and thermally-responsive means having a heating element energized in response to the total current in a part of the excitation system, said thermally-responsive means being adapted to eflect deenergization of the excitation system and disconnection of the generator from the load circuit when heated in response to excessive current in said part of the excitation system.

DONALD W. EXNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA 

